Microphone or receiver of the condenser type



May 30, 1950 w. J. MORELAND, JR 2,509,310

MICROPHONE OR RECEIVER OF THE CONDENSER TYPE Filed Feb. 3, 1948 Patented May 30, 17950 UNITED STATES@ PATENT OFFICE MICROPHONE OR RECEIVER OF THE CONDENSER TYPE William J. Moreland, Jr., Encino, Calif., assignor to George L. Carrington, Encino, Calif., as

agent Application February 3, 1948, Serial-No.5,943

2 Claims.' (C1. 179-111) conductive material stretched to atension which' gives the diaphragm a high natural vibratory frequency. Such a stretched vibratory diaphragm forms one plate of the condenser, the other plate usually being formed byV a rigid bodyspaced a:

small distance from the vibratory diaphragm. The intervening dielectric is usually air. In order. to attain eiciency, the spacing'must be assmall as practicable in proportion toV the. vibrational amplitude of the diaphragm, and if the microphone is acoustically over-loaded the condenserr breaks down by contact or virtual contact' be-y tween the two plates. The diaphragm must be Very evenly stretched in order to attain both efciency and fidelity of response; And all of these requirements result in making such microphones:l

quite expensive.

A general purpose and object of the present invention is to produce a condenser which mayV be used for the various purposes stated, andwhich is comparatively inexpensive to produce'andisi free of breakdown liability. In general, the ina vention typically provides aV condenser structure. in which the vibratory diaphragm utilizesa thinA plate or wafer of a dielectric substance :havinga relatively high modulus of elasticity; such a sube stance as glass.

instance as a thin metallic foil cemented on or a thin layer of metal such as aluminum de-Y posited by the well-known Vacuum-vapor process.

The glass wafer may be and preferably is; made" very thin, and its elastic characteristics give the` diaphragm a high natural vibratoryfrequency.. The spacing between thelinner uncoated'face of the glass wafer and the stationary condenser plate can be very small with 'relation to thevibratory amplitude, resulting in. highefciency; And? if the device` becomes acoustically overloadedtd the point where the dielectric. diaphragm body contacts the xed plate, noelectrical break down` Preferred forms and'designs Vfor the invention.y used as a microphonic transmitter are described"` in detail in the followingk description, asillus'- trative of the invention.. Reference'is h'adto the:4

accompanying drawingsifwhich':

The outer' face of the wafer. carries a thin coating of aA conductor, such for Fig, 1 is a central axial section of a preferred illustrative form of microphone;

Fig. 2 is a fragmentary enlargement of certain portions of Fig; 1;

Fig. 3 is a section taken as indicated by line 3-3on Fig. 1;

Fig. 4 is a schematic fragmentary section illustratingthe relative structure and proportions of two :types of Vmicrophone;

Fig; 5 is a section similar to Fig. 4;.but illustrating a different selection of proportions in the second microphone;

Fig. 6 is an elevation of an alternative form of diaphragm; and

Fig. 7 is a section similar to Fig. 2, but illus,-V

trating a modication.

Referring rst to Figs. l-3, a suitable cylindric case is shown at i9, of any suitable material such as aluminum. One end of the cylindric case is provided with a member which forms an internal shoulder, such as the ring member II which is illustrated as screw-threaded into the outer end" of the casing. Alternatively, the shoulder may be formed. integrally with case I0. A perforated cover I2 may be secured over the shouldered end off case I0.

The vibratory diaphragm I3is formed of a thin waferY I4 of relatively stiff dielectric material, such as glass, quartz or the like, finished by polishing both faces to substantially optical flatness and facialparallelism; with aconductive metal: lic coating I5 applied to its outer face. (SeeFig.

Zior detailed structure of the diaphragm; in Fig.

l the` composite diaphragm is shown as if in edge elevation.) The diaphragml as so formed is preferably of a diameter slightly less than the interior If the metallic coating I5 diameter of case Iii. is deposited by evaporation in vacuum, the circular edge of the diaphragm is shielded from the depositing metal, or, preferably, is ground toI finish size after depositionrof the metal so as to remove any metallic coating on the diaphragm edge. This increases the electrical leakage path between the deposited metal and the xed` electrodel surface, when the latter extends acrossthe entire width of xed plate 2| (see below).

The diaphragm I3 may be pressed' directly against internal shoulder Il of theV case (as in- Fig. '7), with conductive coating I5 thus directlyy electrically connected to the case through metallic ring Il. A preferred structure is that shown in Figs. land 2, in which a ring Il is interposed between diaphragm I3 and shoulder II. Ring I1 is preferably formed of glass or a similar material which can readily be ground andpolishedrto accurate-flatness, so that it will provide uniform 20 may be formed, for example, by Vacuum evaporation of a suitable metal, such as aluminum, and the extent of the layer 2Q can in that instance be controlled as may be desired by the wellknown procedure of masking during the process of evaporation. Layer 20 preferably covers only the central portion of plate body 2l, for reasons to be described. It may, however, cover the whole surface; or, what amounts tc the same thing, plate body 2I may be of metal so that its surface needs no conductive coating. The back of plate body 2I is ribbed as shown at 23 to give the plate rigidity; and the face portion of the plate body is perforated, preferably by a series of concentric slots 24.

A thin spacer ring 23 of suitable easily worked and somewhat resilient material, such for example, as polystyrene, rests on the peripheral portion of the outer face of plate body 2i, and bears directly against the peripheral portion of the inner face of glass wafer I4 in opposition to ring II. Spacer ring 28, which thus determines .the separation between diaphragm I3 and plate body 2l, may be approximately 0.001 inch thick in a typical structure where the glass diaphragm body is about in diameter and also about 0.001 thick.

A backing disk 3l, like plate body 2l, ts snugly but movably within case 20. Backing disk 3| is preferably formed of insulative material, and carries means for applying axial force to plate body 2| and for providing an electrical connection to conductive layer 20 carried thereby. As illustrated, a central metallic plug 35 is screw-threadedly inserted in backing disk 3I and carries pressure screw 36. Plate body 2I also has a central metallic plug 38, the upper portion of which (in the aspect of Fig. l) extends, preferably with reduced diameter as at 38a, just to the upper face of the plate body, where it supports and electrically contacts the central portion of conductive plate layer 20. The lower end of plug 38 either rests upon the upper end of pressure screw 36, or upon a small steel ball 39 which rests on the pressure screw.

When the parts are assembled in the relationships shown, they are clamped in position by retaining ring 40, which is screwthreaded into the lower end of case I0. Pressure is then applied to plate body 2l by pressure screw 36 to press spacer 28 against diaphragm I3 and press the rim portion of the diaphragm firmly against ring Il and shoulder II. Electrical connection to the metallic facing I5 of the diaphragm may be made by connection applied to any part of the metallic casing, and connection to conductive facing 20 of plate 2| by connection to screw 3 or to metal plug 35.

The advantages provided by the invention will be understood from the preceding illustrative description and a consideration of the properties which determine the performance of a microphone in a typical application. In general itis desirable to make the electrical capacitance of the microphone, for equilibrium spacing of the plates, as large as possible; and to make the variation of the capacitance which results from a sound wave of given amplitude as large as possible, not only in absolute value, but, more especially, when expressed as a fraction of the equilibrium capacitance. If Eo is the polarizing voltage applied to the microphone condenser, and Co is the static or equilibrium capacitance, the relative voltage variation dE/E corresponding to a variation of capacitance dC is given by Thus the theoretical voltage variation 02E is directly proportional to the relative capacity variation dC/ Co. For a conventional condenser microphone having an air gap Do and plate area A, a

Accordingly, for highest sensitivity the normal air gap D0 should be small and its variation dD (for some given sound amplitude) should be large. In conventional practice, one of the factors limiting sensitivity is the possibility that momentary acoustical overloading of the microphone may cause the movable plate to contact the fixed plate, resulting in electrical breakdown of the condenser and possible damage.

An important advantage of the present invention is the fact that such electrical break-down is prevented by the presence of a dielectric layer interposed between the two plates which form the condenser. In the present illustrative embodiment, that dielectric layer is the glass wafer i4, which also forms the structural portion of vibratory diaphragm I3. Even if the diaphragm should momentarily contact the fixed plate ZI of the condenser, as may occur under extreme acousticalv overload, dielectric layer IG prevents electrical contact between plates i5 and 2E) of the condenser. Thus the normal air gap between diaphragm I 3 and fixed plate 20 can safely be made appreciably smaller for a diaphragm of given stiffness; or a more exible diaphragm can safely be used with given air gap, leading in either instance to a larger relative variation of capacity ZC/Co, and hence to greater sensitivity.

A further advantage of the present invention is the superior accuracy and uniformity with which a small spacing Du can be established over the whole of the effective area A of the diaphragm. This is most readily accomplished by making both diaphragm I3 and the upper face of plate body 2l (as seen in Fig. 1) accurately iiat. The use of glass as the structural element of the diaphragm makes possible a higher accuracy of figure than can readily be obtained, for example, with a metal diaphragm, since glass can be ground, polished and tested by precision methods familiar to the optical industry. Moreover, glass, when properly selected and worked, is almost completely free of internal strain and inhomogeneities which might lead to irregular vibratory motion or unsymmetrical modes of vibration. When glass or a similar substance is used for the diaphragm, the superior accuracy of gure permits closer and more uniform spacing between diaphragm and fixed plate, and the uniform elastic properties of the glass tend to give superior fidelity of response of the microphone over the wide range of frequencies.

To take full advantage of the inherent precisicn of a glass diaphragm, it is desirable to provide a locating surface for the edge portion of the diaphragm which is of comparable or even greater accuracy. Such a surface is provided, according to the present invention, by the inner face of ring I1, which is made of some material such as glass which can be worked by optical methods. That surface is then not only flat as to its general form, which may be true also of shoulder ll against which the outer 'face of the ring is seated, but it is also fiat in its detailed structure, being highly polished and substantially free of local irregularities. Spacing washer 28 may be worked to similar accuracy of form, but that is not ordinarily necessary if the face of ring l1 is true and if washer 28 is made of a material which is relatively soft or elastic as compared to glass.

Plate body 2|, which carries the relatively fixed condenser plate in the form of conductive layer 2D, is preferably made of some insulative material that can be accurately worked, for example by machining, as Bakelite; or by optical grinding and polishing, as glass. Central metal plug 38 mustbe rigidly seated in plate body 2|, so that the end of plug extension 38a will have a xed relation to the face of the body. That plug end preferably lies accurately in the plane of the body face, but, because of its relatively small diameter, it may be cut slightly back from that face without significantly aecting the capacity or performance of the microphone. The metal of plug 38 and its extension 38a may be chosen to have a temperature expansion coeiiicient the same as or close to that of the plate body material. The upper end of 38a may then be iiatfinished in the operation of finishing the surface of body 2l.

The equations given above are based on the assumption that the gap between the two plates of the microphone condenser is filled by a single medium, ordinarily air. On the other hand, in the preferred form of microphone in accordance with the present invention the space between condenser plates I5 and 20 is filled partly by air and partly by the dielectric material of the body I4 of the diaphragm. For a given free-air space between the fixed and vibratory members, the total space between the actual condenser plates is therefore greater than in a conventional structure. An important feature of the present invention is the recognition that such an increase in physical separation of the condenser plates, where the increase is caused by interposition of a dielectric layer, is of little importance in comparison with the advantages produced by that layer. The reason for this, expressed in general terms, is that such an increase in plate separation does not lead to a proportional decrease in the capacitance, nor in the relative variation of capacitance which results from a diaphragm displacement of given amplitude. The relatively high dielectric constant of the glass portion of the inter-electrode space reduces the effect of that space upon the capacitance to a small factor. For example, given an ordinary condenser with air gap D1, and a condenser of equal area -but having its plates separated by an air gap DA and also by a constant space DG filled with a dilals , 6 electric having dielectric constant k, the ratio of their respective capacitanoes IC1 and C2 is gli gli* kV C2 D1 In both instances the effect of adding to an air gap DA a space DG filled with dielectric 1c is the same as if the air gap itself were increased by an amount Do/k. Since the dielectric constant lc of glass is typically about 6, the effect of adding a glass layer between the condenser plates is only about one sixth the effect of adding an equal space filled with air. For example, in the specific illustrative case of two microphones having glass diaphragms of equal thickness (De) 0.001 and equal elastic properties, the first having a conductive film on the inner face of thediaphragm and the second having a similar conductive film on the outer face of the diaphragm in accordance with the invention; if both microphones have equal air gaps Di and DA of, say, 0.001, as illustrated in Fig. 4, both the capacity and the voltage response of the second microphone will be smaller than that of the first, because of the greater plate separation. But, whereas the actual plate separation of the second microphone is twice that of the first, the decrease of capacity and voltage response is found from Equations 4 and 5 to be less than .15

If the air gap DG of the second microphone is reduced only 17% relative to the air gap D1 of the first, as illustrated in Fig. 5, the capacitance and response of the two will be equal; and that in spite of the fact that the actual plate separation is still greater in the second microphone than in the first. Further reduction of air gap in the second microphone, which is generally feasible for the reasons already discussed, leads to clear gain in operating characteristics.

By selecting a dielectric for the diaphragm body which has a dielectric constant greater than 6, other characteristics remaining the same, for example, the performance of the instrument can be further increased. Such a gain is not great, and the results already described may be considered as typical.

However, my invention is not limited to the use of glass as the body of the diaphragm. Other dielectric substances such as quartz, or porcelains, may be used for that body and for the plate body 2|. Generally speaking, the chosen substance, particularly for the diaphragm, should be -one capable of accurate surfacing. Glass, quartz vand hard porcelains are of that character and also have relatively high moduli of elasticity which is desirable to give the diaphragm a high resonant frequency.

In actual practice, the vibratory motion of a diaphragm supported peripherally is of course not uniform over its yentire area, but is greatest in the central region and decreases to zero at the edge. For this and other reasons, the above analysis, although a useful simplification, particularly when applied to a restricted element vof area of the diaphragm surface, does not give al complete picture of the performance characteristics of a microphone. The primary result of the radial variation in the amplitude of vibration of the diaphragm is that the outer portions of the condenser plates contribute to the total static capacitance Co of the condenser without contributing a fully equivalent amount to the vvari-- ation dC of that capacitance. Thus the relative variation of capacitance dC/Co is smaller for the outer portions of the plate than for the central portion, and the overall value of dC/Co for the entire condenser, which determines the relative voltage variation, is reduced.

This effect is partially avoided by the present invention by limiting one or both of the electrically conductive condenser plates (in the embodiment illustrated in Figs. 1-3, the relatively xed plate 2li) to the central area of the vibratory diaphragm. The outer portions of the diaphragm, where the amplitude of vibration is greatly reduced, then do not affect the value of dC/Co, since they do not contribute appreciably either to dC or to C0. Although the conductive layer I5 on the diaphragm itself extends over the whole of the diaphragm face, it produces negligible electrical capacity except in those portions which are immediately adjacent the relatively small fixed plate 20. A further advantage of limiting the effective condenser area to less than the diaphragm area is that the leakage path between the condenser plates is greatly increased.

The same result is obtained by any structure W'which limits the effective portion of the con'- denser to the central region of the circular vibratory diaphragm. Thus, the conductive layer I5 on the diaphragm itself may be mainly limited to the central region of the diaphragm, and the relatively fixed plate 20 may then be similarly limited, or may extend opposite the whole of the vibratory diaphragm. Connection to such a central conductive layer (see Ia in Fig. 6) can then be made, for example, by coating relatively narrow strips |51) which extend from the central layer to the peripheral portion of the diaphragm |305. The uncoated areas between strips I5b are preferably made fairly narrow to prevent stray fields from disturbing the performance of the microphone; or the cover I 2 may be formed as an electrical shield to reinforce the shielding action of the apertured coating I5a.

If it is preferred to make the relatively fixed plate 2| of solid metal, an alternative structure such as that indicated in Fig. '7 is preferred. Backing plate 3|, which isY of insulative material (Fig. 3), is then formed as shown at I3! in Fig. 7 with an upwardly extending rim |32 which encloses the periphery of metal plate I2I and insulates it from case I0. The outer face of plate |2| can then be cut back to formr a channel as at |26, leaving a central portion |21 closely adjacent the vibratory diaphragm H3 and a narrow rim portion |28 which supports the diaphragm. The capacitance associated with rim portion |28 is small, since the rim is relatively narrow and its surface may be lower than surface |2'i'; and that associated with the channeled area |26 is small because its spacing from vibratory diaphragm I3 is there relatively large. Hence the eective capacitance of the microphone is mainly that associated with central plate portion |21 of plate |2I. Since this lies opposite the central portion of the vibratory plate, where the vibrational amplitude is relatively large, the

eilect of channel |26 is to increase the value of dC/Co in much the same manner that has been described in connection with the limited area conductive layer 20 in Figs. l and 2'.

Fig. 7 illustrates certain other modifications of the embodiment of Figs. 1-3. In Fig. 7 the glass ring Il of Figs. 1 and 2 is omitted, and the diaphragm I|3 is shown as pressed directly against case flange I I, although the glass bearing ring I7 may also be interposed as in Fig. 2. The diaphragm itself in Fig. 7 is again shown as formed. of a thin conductive layer |I5 carried by a diaphragm body III'I, which is again preferably but not necessarily a wafer of such a dielectric materialV as previously discussed; but the conductive layer is in this instance on the inner or lower face of the diaphragm, adjacent plate |2I. In order to provide an insulating or dielectric layer between condenser plates II5 and |2I, a. thin layerl of insulative or high dielectric material Mil is applied to the central portion |21 of plate |2I. This layer I may, for example be a thin Wafer of glass or similar substance cemented to the face of plate |2I. If this wafer has appreciable thickness, spacing ring |29 is thickened correspondingly to maintain the desired free space below the central portion of vibratory diaphragm I I3. This spacing ring may be of any suitable insulative material, e. g. of glass finished as described for ring Il. Electrical connection between layer I land the case may be had, for instance, by extending that layer as at I i5a around the edge of wafer IIi to contact the metallic shoulder ring II.

With the structure just described, the thickness and material of body portion IIfl of diaphragm IIS can be chosen to give the most desirable vibratory properties, Without any regard to the capacity of the microphone condenser, the latter being determined by the air gap and the thickness of protective dielectric layer Ii, which rlray be supported over its entire area by plate I claim:

l'. .en electrical condenser of the type described, comprising a tubular metallic shell having an internal annular shoulder near its outer end, a seating ring of glass-like material Seated against the shoulder and having an inner polished flat face, a vibratory diaphragm in the shell With the peripheral portion of its outer face engaging the inner face of the seating ring, said diaphragm comprising a radially unstressed thin wafer of a glass-like material having polished fiat parallel faces and a thin layer of conductive material on its outer face, an annular spacer lying against the peripheral portion of the inner diaphragm face, a relatively xed plate comprising a body of dielectric material having an outer flat surface with its peripheral portion bearing against the annular spacer and a thin layer of conductive material on, and restricted to the central portion of, its outer iiat surface, and means secured in the inner end of the case pressing against the inner face of the ternal annular shoulder near its outer end, a seating ring seating against the shoulder, a vibratory diaphragm in the shell with the peripheral portion of its outer face engaging the inner face of the seating ring, said diaphragm comprising a radially unstressed thin wafer of a glass-like material having polished fiat parallel faces and a thin layer of conductive material on its outer face, an annular spacer lying against the peripheral portion of the inner diaphragm face, a relatively Xed plate comprising a body of dielectric material having an outer flat surface with its peripheral portion bearing against the annular spacer and a thin layer of conductive material on, and restricted to the central portion of, its

outer flat surface, and means secured in the in- 15 ner end of the case pressing against the inner face of the fixed plate to press it against the glass-like Wafer to hold the Wafer against the 10 seating ring and the seating ring against the shoulder, the bearing of the wafer against the seating ring and of the spacer ring against the Wafer being such as not to appreciably stress the Wafer in its own plane.

WILLIAM J. MORELAND, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,239,852 Vreeland Sept. 11, 1917 1,456,538 Crandall May 29, 1923 1,550,381 Massole et al. Aug. 18, 1925 2,086,107 Wilson July 6, 1937 

